1. Field of the Invention
This invention relates to continuously-variable-ratio transmissions ("CVT's.revreaction.) of the toroidal-race rolling-traction type, and in particular to roller-control units of the kind by which the orientation of the traction-transmitting rollers may be controlled hydraulically, by means of the direct connection of the carriage of each roller to a piston, movable within a hydraulic cylinder connected to a controlled source of variable fluid pressure.
2. Description of the Prior Art
Several patent publications of recent years, of which WO90/05860 is an example, describe toroidal-race transmissions in which direct connection of each roller to a piston subject to controlled hydraulic pressure suffices to maintain the angular orientation of each roller--and thus the ratio transmitted by it from an input disc to an output disc--at the appropriate value at all times. The key to the efficacy of such an apparently simple method of roller control is that for any given combination of the essential parameters (operator demand, engine load and speed, final output speed etc.,) of the complete driveline of which the CVT is part, for each angular orientation that each roller may take up within its possible range of such orientations there will be a unique torque loading to which that roller will be subjected. A control system receives inputs representative of all the essential parameters, and sets up within the cylinder the appropriate hydraulic pressure to match the torque reaction associated with the desired orientation, so causing the roller to seek and hold that orientation.
One of the problems encountered in practice by CVT's, in which the rollers are so controlled, is that whereas the centre of a simple piston executes a straight-line motion as that piston reciprocates within its cylinder, the motion required of the centre of a roller of a toroidal-race CVT, as that roller changes its orientation, is not straight-line. Instead, the roller centre must be able to travel a short distance back and forth along the circumference of the centre circle of the imaginary torus, to whose outline the surfaces of the co-operating races formed on the input and output discs must correspond. The roller centre must therefore move along an arc. To accommodate this, in most of the embodiments described in publication WO90/05860 the sealing ring of the piston has a part-spherical outer face, allowing the piston to tilt slightly within the cylinder without losing sealing contact with it. Such a design, although practicable, requires more careful manufacture and assembly than a simple piston, and in particular requires a special seal where the piston rod passes through the cylinder end, to accommodate the wobbling of the rod which results from the tilting of the piston. Also, where the piston Is double acting (as it is in several of the embodiments in WO90/05860) it is practically difficult to avoid inequality of area for the opposite faces of the piston, because a dummy rod, mounted on the rear face of the piston, would require the complexity of a second special seal where It passed through the rear end wall of the cylinder.
Patent Specifications GB-A-1600972 and GB-A-1600973 both relate to CVT's of the toroidal-race rolling-traction type, and show examples of their respective inventions in which the rollers are controlled hydraulically and each roller is connected to a non-tilting piston by a link connected to that piston by a simple pivoting joint. However, all those examples have two significant features in common. Firstly the pistons are single-acting only, which is all they need to be in the alternator unit, or other aircraft accessory, to which the inventions of these two publications are primarily directed. Secondly, in those examples the contact between the piston and the cylinder does not constitute the only locus of effective contact between each roller and the fixed structure of the CVT, because the link connecting each roller to its piston also extends away from the roller on the side remote from the piston, and a spigot at the extremity of that remote side of the link engages with and Is constrained by a fixed slot.